1. Field of the Invention
The present invention relates to a packet communication system and, in particular, the present invention relates, in a shared medium type point-to-multi point communication system in which a plurality of terminal side devices are connected to one network side device through a physical medium, to an assigning technology for dynamically assigning an identifier for identifying an up link time slot allocated by the network side device to an assigning object housed in each terminal side device.
2. Description of the Related Art
As a system with which an access network may be realized at low cost, there is a shared medium type point-to-multi point communication system in which a network side device is connected to a plurality of terminal side devices through a shared physical medium. FIG. 12 shows ATM-PON (Asynchronous Transfer Mode-Passive Optical Network) system, which is an example of the shared medium type point-to-multi point communication system and which includes, for example, four terminal side devices. As shown in FIG. 12, the ATM-PON system includes a plurality of terminal side devices 1200 to 1230, which are connected to a network side device 1240 through an optical splitter/coupler 1260. In such ATM-PON system, in order to avoid collision of data (referred to as “cell”, hereinafter) on a transmission line (referred to “shared transmission path”, hereinafter) between the optical splitter/coupler 1260 and the network side device 1240, which is commonly used by all of the terminal side devices 1200 to 1230, the respective terminal side devices 1200 to 1230 transmit cells to the network side device 1240 according to a time slot information allocated thereto by the network side device 1240.
A cell transfer operation from each of the terminal side devices 1200 to 1230 to the network side device 1240 in the ATM-PON system will be described with reference to FIG. 12.
The network side device 1240 includes a time slot allocator 1242 for allocating time slots to the respective terminal side devices 1200 to 1230 and an identifier assignor 1241 for assigning identifiers for identifying the allocated time slot of every terminal side device (referred to as merely “identifiers”, hereinafter) to the terminal side devices 1200 to 1230. Further, it is assumed that the terminal side device 1200 is constructed with a buffer 1201 for accumulating input cells from respective terminals 1280 and 1281 and an output control portion 1202 for controlling outputs of the cells in the buffer 1201 according to the time slot information notified by the network side device 1240. Incidentally, it is assumed that the respective terminal side devices 1210, 1220 and 1230 have identical constructions to that of the terminal side device 1200. The network side device 1240 is connected to the terminal side devices 1200 to 1230 through the shared transmission path 1270 between the network side device 1240 and the optical splitter/coupler 1260 and individual transmission paths 1250 to 1253 between the optical splitter/coupler 1260 and the terminal side devices 1200 to 1230.
In an initial setting, the identifier assignor 1241 of the network side device 1240 assigns the identifiers to the respective terminal side devices 1200 to 1230 and transmits the assigned identifiers to the output control portions 1202 of the respective terminal side devices 1200 to 1230 through a control signal 1291. Each of the output control portions 1202 holds the identifier assigned thereto and uses it when it outputs a cell in a usual state.
Now, the usual state will be described.
The time slot allocator 1242 of the network side device 1240 allocates the time slots to the respective terminal side devices 1200 to 1230. And then, the time slot allocator 1242 obtains an identifier information of the respective terminal side devices 1200 to 1230 from the identifier assignor 1241 and describes the identifiers of the terminal side devices 1200 to 1230 corresponding to the allocated time slots. The time slot allocator 1242 notifies the output control portions of the respective terminal side devices 1200 to 1230 of the time slot information including described identifiers by means of a control signal 1290. Each output control portion 1202 receives the time slot information from the network side device 1240 and, when the described identifier is coincident with the identifier assigned thereto, outputs the cell in the buffer 1201. In the buffer 1201, input cells from the terminals 1280 and 1281 are accumulated and the input cells are outputted to the network side device 1240 by the output control portion 1202. The cells outputted from the terminal side devices 1200 to 1230 are transferred to the network side device 1240 through the individual transmission paths 1250 to 1253, the optical splitter/coupler 1260 and the shared transmission path 1270 and, further, transferred to a station device through a transmission path 1292.
As described above, in the ATM-PON system, the terminal side devices 1200 to 1230 having identifiers assigned by the identifier assignor 1241 of the network side device 1240 can transmit data cells to the network side device 1240.
However, there are problems in such system.
A first problem is that, since the number of terminal side devices, which can transmit data cells to the network side device, is limited to a total number of identifiers, it is impossible to efficiently accommodate the terminal side devices when the number of the terminal side devices exceeds the total number of the identifiers. This will be described in more detail.
In FIG. 12, it is assumed that three identifiers #A, #B and #C are set for the four terminal side devices 1200 to 1230. FIG. 13 shows an example of the assigning states 1300, 1310, 1320 and 1330 of the identifiers to the respective terminal side devices 1200, 1210, 1220 and 1230 and the working states 1301, 1311, 1321 and 1331 thereof in time series under the above condition. Each of the assigning states 1300 to 1330 includes an assigned state and a not assigned state and, in the assigned state, an identifier assigned is shown. In each of the working states 1301 to 1341, there are an ON state in which a communication to the network side device 1240 is being performed and an OFF state in which a communication to the network side device 1240 is not performed due to non-use period or cutoff of a power source.
In the initial setting, the identifiers #A, #B and #C are assigned to the terminal side devices 1200, 1210 and 1220, respectively, and no identifier is assigned to the terminal side device 1230. Now, it is assumed that the working state 1321 of the terminal side device 1220 is turned from the ON state to the OFF state at a time instance T and, thereafter, the working state 1331 of the terminal side device 1230 is turned from the OFF state to the ON state at a time instance T′. Even in such case, the identifiers #A, #B and #C are fixedly assigned to the terminal side devices 1200, 1210 and 1220, respectively. Therefore, no identifier is assigned to the terminal side device 1230 whose working state becomes the ON state after the time instance T′, so that the terminal side devices 1230 can not transmit data cell to the network side device 1240.
Since the identifiers are fixedly assigned to the respective terminal side devices in the conventional system as mentioned above, the terminal side devices to which no identifiers are assigned in the initial setting step can not transmit data cells to the network side device and the number of the terminal side devices, which can transmit data cells to the network side device is limited to the total number of the identifiers. Consequently, the conventional system can not efficiently accommodate the terminal side devices when the number of the terminal side devices exceeds the total number of the identifiers.
A second problem of the conventional system in which the identifiers are fixedly assigned to the terminal side devices will be described. When an identifier is assigned to a group of connections accommodated in each of the terminal side devices, there may be a case where objects to be assigned can not be efficiently accommodated even if the total number of identifiers is larger than the number of the objects to which identifiers are assigned.
For example, it is assumed in FIG. 12 that the total number of identifiers is 4 and the number of the objects to which identifiers are assigned and which are accommodated in the whole system is 3. In the conventional system, one identifier is fixedly assigned to each of the terminal side devices 1200 to 1230. When three objects to which identifiers are assigned are accommodated in one terminal side device (for example, the terminal side device 1200), no identifier is assigned to two of the three objects, so that these objects can not transmit data cells to the network side device.
As such, when the number of objects accommodated in each of the terminal side device is not uniform in the conventional system in which the identifiers are fixedly assigned to the respective terminal side devices, there may be objects to which no identifier is assigned and which can not transmit data cells to the network side device. Therefore, it is impossible to efficiently accommodate the objects even if the total number of identifiers is larger than the number of the objects.